The present invention relates to a multi-window apparatus for displaying a motion video image, a photograph, a character image and the like on the same screen. More particularly, the invention relates to a multi-window apparatus in which a window for displaying a motion video image or a photograph is provided with an expanded range of display luminance to obtain a very realistic image, and a window for displaying a character image or the like displays an image easy to see.
A multi-window apparatus is an apparatus for displaying a plurality of images on a single screen.
FIG. 21 shows a specific example of the configuration of such a multi-window apparatus. The reference numeral 201 denotes a central processing unit, 202 a main memory, 203 a graphics control means, 204 a video memory, 205 an image data conversion means, and 206 a display unit.
The central processing unit 201 reads a program from the main memory 202 and executes it, thereby giving instructions to the graphics control means 203 for the input/output of image data, controlling the image data conversion means 205, and also controlling other operations.
The main memory 202 stores programs to be executed by the central processing unit 201 and also stores image data. A plurality of image data are stored in the video memory 204 which outputs the image data as an image data digital signal 20a.
The graphics control means 203 receives instructions for the input/output of image data from the central processing unit 201, thereby controlling the video memory 204.
In the image data conversion means 205, the image data digital signal 20a output from the video memory 204 is subjected to color conversion and is also converted into a data form suitable for the display unit 206. The details of these conversion processes are set by the central processing unit 201.
FIG. 22 shows the configuration of the above-mentioned image data conversion means 205. The image data conversion means 205 consists of a color conversion means 211 and a D/A conversion means 212. The color conversion means 211 receives the image data digital signal 20a and performs color conversion of the image data, thereby outputting a color-converted image data digital signal 21a. The D/A conversion means 212 receives the color-converted image data digital signal 21a and converts it from digital to analog form, thereby outputting a display data analog signal 20b.
It is herein assumed that the image data digital signal 20a input to the image data conversion means 205 carries three sets of 8-bit data (24 bits in total), each representing one of the three primary colors: red (hereinafter referred to as "R"), green ("G") and blue ("B").
The color conversion means 211 has a color-correspondence table for each of the colors R, G and B, containing data on the color conversion of each color. Each color-correspondence table has 256 (=2.sup.8) entries. In each entry, color data to be obtained after color conversion is set. In accordance with the three sets of data respectively representing R, G and B in the input image data digital signal 20a, three sets of color data to be obtained after color conversion are respectively read out from the entries of the three color-correspondence tables, and then output as the color-converted image data digital signal 21a. The contents of the color-correspondence tables are set in accordance with a table update signal 20c sent from the central processing unit 201.
There is also a multi-window apparatus in which a color conversion means has a plurality of color-correspondence tables for each color, so that display images in different windows are allowed to have different color tones (Japanese Laid-open Patent Publication Nos. 60-209786 and 62-136695).
The D/A conversion means 212 converts the color-converted image data digital signal 21a from digital to analog form, and outputs the result of the conversion as the display data analog signal 20b.
An image is displayed as follows: In accordance with an instruction given by the central processing unit 201, image data are input to the video memory 204 by the graphics control means 203. The image data stored in the video memory 204 are read out as the image data digital signal 20a under the control of the graphics control means 203, and then input to the image data conversion means 205. In the image data conversion means 205, the image data digital signal 20a is subjected to data conversion, and then output as the display data analog signal 20b to the display unit 206, where the image is displayed accordingly.
A conventional multi-window apparatus such as described above, however, involves the following problem.
Since the image data digital signal carries three sets of 8-bit data respectively representing R, G and B, the luminance of each color is represented by a value of 0 to 255 (=2.sup.8 -1). The value "0" represents the lowest luminance value, while "255" represents the highest luminance value.
FIG. 23 shows the pixel distribution with respect to luminance in each set of image data. The solid line indicates the pixel distribution of an image which is produced by using a camera (hereinafter referred to as a "camera image"). The broken line indicates the pixel distribution of an image which is produced by computer (hereinafter referred to as a "CG image"). Camera images include photographs of landscapes or people, motion video images, and the like. CG images include character images, images created by so-called computer graphics, and the like.
For example, in the case of a character image, for the purpose of making the image easy to see, high contrast between characters and the background is often provided by, for example, allowing black characters to be displayed on the white background. Thus, in the pixel distribution with respect to luminance in the character image, a large number of pixels are concentrated in the vicinity of the highest luminance value (255) and in the vicinity of the lowest luminance value (0), as shown by the broken line of FIG. 23. On the other hand, in the case of a camera image, objects with various levels of brightness are used as the image data to be displayed. Accordingly, in the pixel distribution of the camera image, a large number of pixels are concentrated at intermediate luminance values, as shown by the solid line of FIG. 23.
In cases where such camera and CG images as described above are displayed on the same screen, the camera image becomes relatively dark if the brightness of the screen is set so as to make the CG image easy to see. Conversely, if the brightness of the entire monitor screen is increased to make the camera image easy to see, the contrast of the CG image becomes too high, which makes the CG image too bright and accordingly makes it difficult to see. The reason for this is that the CG image and the camera image require different levels of screen brightness in order to be clearly seen by the human eye. The optimum screen brightness for the camera image is of such a level that the pixels with the intermediate luminance values are allowed to be clearly seen. On the other hand, the optimum screen brightness for the CG image which is suitable for the human eye is of such a level that the pixels with luminance values in the vicinity of the highest luminance value (255) are not too bright and can be seen clearly.
As described above, in the conventional multi-window apparatus, when a camera image and a CG image are displayed on the same screen, it is impossible to obtain both the optimum levels of screen brightness which allow the camera and CG images to be easily seen. Most users adjust the brightness of the monitor in accordance with the luminance of the characters. This causes a defect in the conventional multi-window apparatus; the camera image becomes dark and difficult to see.